drm/panfrost: Fix panfrost device variable name in devfreq

Commit 64111a0e ("drm/panfrost: Fix incorrect updating of current
device frequency") was a Panfrost port of a similar fix in Panthor.

Fix the Panfrost device pointer variable name so that it follows
Panfrost naming conventions.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: 64111a0e ("drm/panfrost: Fix incorrect updating of current device frequency")
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250520174634.353267-6-adrian.larumbe@collabora.com